Conductive spacer in a via

ABSTRACT

A method of constructing a conductive via spacer within a dielectric layer located between a first metal layer and a second metal layer includes the steps of depositing a conductive spacer layer within the opening and over the first metal layer. A portion of the conductive spacer layer is removed to leave a conductive spacer within the opening. The second metal layer is deposited over the spacer to complete the connection between the first and second metal layers. The spacer preferably comprises a material selected from the group comprising refractory metal silicides and nitrides. The spacer is preferably tapered and the via may include a glue layer to improve the adherence of the spacer to the dielectric layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed generally to a via containing a spacerand a method of making the via, and, more particularly, to a viacontaining a conductive spacer.

2. Description of the Background

It is well known in the semiconductor art to use interconnectstructures, known as vias and contacts, to connect an upper conductor ofcurrent, such as metal or polysilicon, through a dielectric layer to alower conductor of current. A via is an electrical connection betweentwo metal layers, and a contact, in contrast, is an electricalconnection between anything other than two metal layers, such as betweenmetal and silicon. Vias and contacts are used extensively in very largescale integrated (“VLSI”) circuits, and an average VLSI circuit maycontain 16 million vias and contacts.

Vias and contacts are formed by an opening in a dielectric layer and aconductor within the opening. Directional deposition methods, such asevaporation and sputtering, are often used to deposit the conductorwithin the opening. Such methods, however, often provide poor stepcoverage and only a thin conductive layer on the vertical wall of theopening. Thin layers are often not sufficient to provide good electricalcontact between the upper and lower conductors, and result in a highresistance and a propensity for electromigration failures.

Although non-directional deposition methods, such as chemical vapordeposition, provide good coverage of vertical walls, those methods donot provide satisfactory results with conductors which are preferablyused in vias and contacts, such as aluminum.

It is well known that step coverage can be greatly improved indirectional deposition methods by tapering the wall of a via or contact,so that the wall is not substantially vertical and the diameter at thetop is greater than the diameter at the bottom.

One approach to taper a wall is by sputtering or etching the walls ofthe opening, as disclosed, for example, in U.S. Pat. NO. 5,269,880,issued to Jolly et al. That approach, however, often requires additionalprocess steps to provide the bottom of the opening with a protectivelayer prior to sputtering or etching, and then removing the protectivelayer following sputtering or etching, as disclosed in the Jolly et alpatent.

An alternative approach is to taper the walls by providing a spacerwithin the via or contact, without disturbing the dielectric layer andwithout the need for a protective layer to protect the bottom of theopening. An example of such a method is disclosed in U.S. Pat. No.4,489,481, issued to Jones. Spacers are typically used in criticalgeometries where dimensions, such as the size of the opening, must beprecisely controlled.

Dielectric materials have traditionally been used as spacers, asdisclosed in the Jones patent. Oxides, for example, have a smoothsurface which allows more precise control of their dimensions. Metals,in contrast, have a “grainy” structure which results in a rough surface.The rough surface, in turn, makes it difficult to precisely control thedimensions of a metal structure. As a result, it was believed thatmetals were not suited for use in critical geometries, such as spacers,and so the use of metal as a spacer has generally been ignored.

In addition, vias and contacts are typically formed in oxide layers, andwhile oxide spacers adhere very well to the oxide layer, many metals donot, and thus require an intermediate “glue” layer, such as polysilicon,which adheres well to both oxide and the metal.

Spacers made of a dielectric, however, have the undesirable property ofbeing an insulator. As a result, the use of a dielectric spacer narrowsthe contact area between the conductor in a via or contact and the upperor lower conductor, resulting in increased resistance. Furthermore, whena dielectric spacer is etched, it leaves dielectric material in thebottom of the opening, which further increases the resistance of the viaor contact, or alternatively, requires additional process steps toremove the dielectric material.

Those problems are exaggerated as the density of integrated circuitsincreases and the size of openings decreases. As the diameter of anopening approaches one micron, the aspect ratio approaches 1 to 1, anddirectional deposition methods, even when used in conjunction withtapered sidewalls or dielectric spacers, do not provide a reliableelectric contact. In those situations, to reliably obtain goodelectrical contact the opening is usually completely filled with aconductor, known as a “plug”. Plugs, however, are time consuming toproduce, and they require several additional process steps. Furthermore,shadowing effects during the formation of plugs often limit theireffectiveness as a good electrical connection. As a result, the use ofplugs is not always a desirable option.

U.S. Pat. No. 4,507,853, issued to McDavid, discloses the use of metalspacers in a contact between a metal layer and a silicon substrate.There is no disclosure in the McDavid patent, however, of a method offorming a metal spacer in a via. The method disclosed in the McDavidpatent does not address the problems present in forming a metal spacerin a via. For example, one problem with forming a metal spacer in a via,which is not addressed in the McDavid patent, is determining when tostop the etch process used to form a spacer from a metal layer depositedin an opening. In the process disclosed in the McDavid patent, a siliconsubstrate acts as an etchstop. When forming a metal spacer in a via,however, a process that etches the metal layer used to form the spacerwill also etch an underlying metal layer. The etch process, therefore,must be stopped before it reaches the underlying metal layer. In anextreme case, the etch process can destroy the underlying metal layer.In a less extreme case, the surface of the underlying metal layer willbe recessed by the etch process, increasing both the effective depth andthe aspect ratio of the opening, making it more difficult for asubsequent metal layer to make contact with the underlying metal layer.In contrast, if the etch process is stopped too soon, the removal of theconductive layer will not be effective, and a thin film of conductorwill remain where it is not wanted.

Thus, the need exists for via spacer with improved electromigrationproperties, with improved metal step coverage, and with reducedresistance, thereby allowing the use of smaller vias and more densesemiconductor devices.

SUMMARY OF THE INVENTION

The present invention is directed generally to a via spacer and a methodof making the same. The via spacer is formed within an opening in adielectric layer between a first and a second metal layer. The inventionincludes the step of depositing a spacer layer within the opening of thevia so that the spacer layer is in contact with the first metal layer.The invention also includes the step of removing a portion of the spacerlayer to leave a spacer within the opening, with a portion of the spacercovering the first metal layer. The second metal layer is deposited overthe spacer to complete the connection between the first and second metallayers.

The spacer preferably comprises a material selected from the groupcomprising refractory metal silicides or their associated nitrides, suchas, for example, titanium silicide, titanium nitride, and cobaltsilicide. The spacer is preferably tapered and the via may include aglue layer to improve the adherence of the spacer to the dielectriclayer.

The invention solves the above-mentioned shortcomings in the prior artby providing a method of forming a conductive spacer, which provides forgood step coverage and which does not decrease the contact area of thevia.

BRIEF DESCRIPTION OF THE DRAWINGS

For the present invention to be clearly understood and readilypracticed, the present invention will be described in conjunction withthe following figures, wherein:

FIGS. 1, 2, 4, and 5 are cross-sectional views of a via of an integratedcircuit at successive steps in a method of fabricating a via spaceraccording to the present invention:

FIGS. 3 illustrates an alternative embodiment of a via constructedaccording to the teachings of the present invention;

FIG. 6 and 7 illustrate another embodiment of a via constructedaccording to the teachings of the present invention;

FIG. 8 illustrates another embodiment of a via constructed according tothe teachings of the present invention; and

FIG. 9 illustrates a system in which the present invention may be used.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It is to be understood that the figures have been simplified toillustrate only those aspects of a via which are relevant, and some ofthe dimensions have been exaggerated to convey a clear understanding ofthe present invention, while eliminating, for the purpose of clarity,some elements normally found in a via. Those of ordinary skill in theart will recognize that other elements and process steps are required toproduce an operational via. However, because such elements and processsteps are well known in the art, and because they do not further aid inthe understanding of the present invention, a discussion of suchelements is not provided herein.

FIG. 1 shows an opening 10 formed in a dielectric layer 12, such assilicon dioxide. The construction of openings 10 is well known, and maybe accomplished, for example, by an anisotropic etch using CF₄+CHF₃ in areactive ion etch (“RIE”) reactor, at between 10° C. and 40° C. Belowthe dielectric layer 12 is a silicon substrate layer 13 and a firstmetal layer 14, such as aluminum, which defines the bottom of theopening 10. The metal layer 14 is patterned to form conductors as isknown. The top of the silicon substrate layer 13 and the bottoms of thedielectric layer 12 and first metal layer 14 are separated by aninterface 15.

FIG. 2 shows the structure of FIG. 1 after the application of aconductive spacer layer 16. In the preferred embodiment, the spacerlayer 16 is titanium. Titanium is resistant to electromigration, hasgood “glue” properties with respect to the preferred dielectricmaterial, and has good conduction properties. Although any metal willwork as a spacer, refractory metals, such as titanium silicide, titaniumnitride and cobalt silicide, are preferred because of their superiorglue and electromigration qualities. The spacer layer 16 preferably hasa thickness of between approximately ten percent to forty percent of theopening's diameter, and may be applied using a chemical vapor deposition(“CVD”) process, preferably using a titanium-chloride chemistry, such astitanium and CFCl₃.

Alternatively, titanium may be sputtered directly onto the dielectriclayer 12 and the opening 10. Because sputtered titanium may result inshadowing, a columniator may be utilized to provide a more uniformdistribution of the spacer layer 16, in particular with respect to thewalls and the bottom of the opening 10.

Another preferred material for the spacer layer 16 is titanium silicide,TiSi_(x), deposited by a CVD process. Titanium silicide CVD produces aconformal layer of titanium silicide, yielding uniform metal layersalong the vertical and horizontal surfaces in and around the opening 10.The titanium silicide CVD process uses titanium tetrachloride, TiCl₄, inconjunction with silan, DCS, or some other source of silicon in the gasmolecule.

FIG. 3 shows an alternative embodiment in which a “glue” layer 17 isused. The glue layer 17 may be used, for example, when tungsten is thespacer layer 16, because tungsten does not adhere well to oxides. As aresult, one or more glue layers 17 are used to firmly attach thetungsten to the dielectric layer 12. A preferred glue layer 17 istitanium, which helps to reduce contact resistance by consuming silicondioxide that might remain on top of the first metal layer 14. Anotherpreferred glue layer 17 is titanium nitride, which is much moreconformal and easier to deposit in the opening 10 than titanium, butwhich does not consume silicon dioxide. Polysilicon also works well as aglue layer 17, but has a higher resistance than titanium and titaniumnitride.

FIG. 4 shows the opening 10 after the spacer layer 16 has been etched,leaving a spacer 18. The spacer 18 is tapered, resulting in the opening10 being wider at the top than at the bottom. The etch is preferably ananisotropic etch performed, for example, using a chlorine-based gas,such as Cl₂, CCl₄, or HCl, which is an effective etch for titanium. Theetch may also use BCl₃, SiCl₄, and Cl₂, which is an effective etch foraluminum, titanium nitride, titanium silicide, and polysilicon.

The anisotropic etch removes the spacer layer 16 from horizontalsurfaces. The etch also removes material more quickly from the top ofthe dielectric layer 12 than from the bottom of the opening 10, so thatwhen the spacer layer 16 has been removed from the top of the dielectriclayer 12, there is still a thin spacer layer 19 remaining on the bottomof the opening 10 and over the first metal layer 14. As a result, thespacer layer 16 can be removed from the dielectric layer 12, and damageto the first metal layer 14 can be avoided, by using the removal of thespacer layer 16 from the dielectric layer 12 as a limiting factor. Forexample, the time required to etch the spacer layer 16 from thedielectric layer 12 can be calculated, given the material of the spacerlayer 16, the thickness of the spacer layer 16, and the etch chemistryused. Alternatively, a spectrum analyzer may be used to sense thepresence of the dielectric material, such as silicon dioxide, which isindicative that the spacer layer 16 has been removed from the top of thedielectric layer 12, and the dielectric layer 12 is being etched. Thoseprocesses will have a margin of error created by the spacer layer 19remaining at the bottom of the opening 10 after the spacer layer 16 hasbeen removed from the dielectric layer 12.

Since the spacer layer 19 in the bottom of the opening 10 is aconductor, its presence is not detrimental to obtaining a goodelectrical contact. In fact, the titanium spacer layer 19 cleans thefirst metal layer 14 and decreases the contact resistance by consumingsilicon dioxide which may be present at the bottom of the opening 10. Asa result, it is preferable to have at least a small titanium layer 19remaining in the bottom of the opening 10.

FIG. 5 shows the via after a second metal layer 22 is deposited over thespacer 18. The second metal layer 22 is preferably aluminum deposited bya sputtering process, as is well known in the prior art. Othermaterials, of course, may be used for the second metal layer 22, such ascopper, titanium, tungsten, and various alloys, and they may bedeposited in any manner, such as by sputtering and CVD. The second metallayer 22 will also be patterned to form discrete conductors (not shown).

FIG. 6 shows an alternative process wherein the spacer 18 is formed bysputter etching the spacer layer 16, resulting in tapered corners 20 andadditional spacer material at the bottom of the opening 10, forming apartial plug 21. The partial plug 21 has a thickness that is between tenpercent and forty percent of the via's depth, and effectively reducesthe depth of the opening 10 and the aspect ratio. The combination of ametal spacer 18 and a partial plug 21 greatly increase the step coveragein the opening 10. The angle of the sputter etch is preferably between30 and 60 degrees above horizontal, and preferably uses an argon plasma.The sputtering may leave a portion of the spacer layer 16 on top of thedielectric layer 12, which may be removed immediately by, for example,the same methods discussed above with respect FIG. 4. Preferably,however, the spacer layer 16 is removed at a later time, as discussedbelow with respect to FIG. 7.

FIG. 7 shows a second metal layer 22 deposited on the alternativeembodiment shown in FIG. 6. The second metal layer 22 exhibits good stepcoverage over the spacer 18 and partial plug 21. The second metal layer22 covers the spacer 18, the partial plug 21, and the spacer layer 16 ontop of the dielectric layer 12. Both the second metal layer 22 and thespacer layer 16 are removed at the same time by, for example, theprocess discussed above with respect to FIG. 4 for removing the spacerlayer 16.

As seen in FIGS. 5 and 7, a via constructed according to the presentinvention has excellent step coverage. In addition, the use of a metalspacer 18 acts as a conductor reducing the resistance of the via.Furthermore, because the spacer 18 is a conductor, its dimensions do notneed to be precisely controlled within the opening 10. In fact, superiorresults are achieved if, as in the preferred embodiment, the spacerpartially or completely covers the first metal layer 14. Those featuresallow the via 10 to be smaller, while still providing good electricalcontact and reduced risk of electromigration failure.

FIG. 8 shows another alternative embodiment of the present invention.Because the spacer 18 is a conductor, step coverage of the second metallayer 22 is not required, and so the spacer 18 does not need to betapered. Furthermore, since step coverage is not required, the secondmetal layer 22 may be made thinner, and must only make good contact withthe spacer 18. The conductive spacer 18 forms a ring in the opening 10,and alone is a sufficient conductor between the first and second metallayers 14 and 22.

FIG. 9 illustrates a system 24 in which the present invention may beemployed. The system is comprised of a solid state device, such as amemory device 26, on which connections of the type disclosed herein aremade. The memory device 26 is under the control of a microprocessor 28which may be programmed to carry out particular functions as is known inthe art.

Those with ordinary skill in the art will recognize that manymodifications and variations of the present invention may beimplemented. The foregoing description and the following claims areintended to cover all such modifications and variations.

What is claimed is:
 1. A via carried in an opening having a top and abottom in a dielectric layer of an integrated circuit and locatedbetween a first metal conductor defining the bottom of the opening and asecond metal conductor at the top of the opening, comprising: aconductive spacer in contact with both the first and second metalconductors, said conductive spacer having a lateral portion having athickness formed on an inner wall of the opening and a bottom portionhaving a thickness in contact with the first metal conductor, whereinthe thickness of the bottom portion is less than the thickness of thelateral portion adjacent the bottom portion, and wherein the thicknessof the lateral portion of said spacer is greater near the bottom of theopening than near the top of the opening.
 2. The via of claim 1, whereinsaid spacer comprises a refractory metal silicide.
 3. The via of claim2, wherein said refractory metal silicide comprises titanium silicide.4. The via of claim 2, wherein said refractory metal silicide comprisescobalt silicide.
 5. The via of claim 1, wherein said spacer comprisestungsten.
 6. The via of claim 1, further comprising a glue layer betweenone of the first and second metal conductors and said spacer.
 7. The viaof claim 6, wherein said glue layer comprises titanium.
 8. The via ofclaim 6, wherein said glue layer comprises titanium nitride.
 9. The viaof claim 6, wherein said glue layer comprises polysilicon.
 10. The viaof claim 1, wherein said spacer comprises a refractory metal nitride.11. The via of claim 10, wherein said refractory metal nitride comprisestitanium nitride.
 12. The via of claim 1, wherein said spacer comprisestitanium.
 13. The via of claim 1, further comprising a plurality of gluelayers between one of the first and second metal conductors and saidspacer.